The present invention relates to surge protecting units, and more specifically to circuit and system configurations which provide transient protection for communications modules.
In some electronic systems, such as telephone interface circuits, different system components require different levels of overvoltage and/or overcurrent protection. For example, many SLIC (subscriber line interface circuit) boards are multifunctional, and must interface not only to lines with internally or externally generated ring signals, but also to lines with analog voice and/or high-speed data digital subscriber line.
The boards themselves have a different interface for each of the functions. Each of these interfaces can have a different damage threshold (i.e. each can be damaged by different voltages). As a result, one common level protection with a single protection voltage is unsuitable to protect these cards.
The conventional method of protecting such SLIC based electronics is to place a number of shunt protective elements in the circuit with the desired protection level. Each element is placed immediately adjacent to the interface it is to protect. This technique is shown in FIG. 1.
FIG. 1 illustrates a conventional method of protection. Two terminals (here conventionally labeled Tip and Ring) are fed through respective line feed resistor components LFR, which may include a fusible link. A first protection stage 110, which in this example is implemented by a pair of breakdown diodes, provides a first layer of protection for the LCAS relay (line card access system relay) 120. The LCAS 120 will connect the SLIC interface to the Tip and Ring terminals in the off-hook condition, or disconnect it in the on-hook condition.
When a telephone line receives an incoming call, the ring voltage seen by the LCAS can be 100V above the DC voltage otherwise seen, so that the first protection stage 110 must not suppress normal ring voltages. However, the SLIC interface 140 may not be able to tolerate such high voltages, so the second protection stage 130 may need to be set for a much lower protection voltage. and the SLIC is normally protected from ring voltage by the relay. Thus the second shunt protection stage 130 preferably sets a maximum voltage (for off-hook conditions) which is much lower than that set by the first shunt protection stage 110. As more complex functions are added into telephone line interfaces, other voltage protection values may be needed for other interface elements.
One of the basic design requirements of a robust electrical system is protection against out-of-specification electrical conditions of many kinds, which can arise from many causes. These may include power surges, transient overcurrents, and voltage spikes corresponding to various values of transient energy and source impedance. A variety of protection components have been proposed.
For example, one component is a metal oxide varistor (MOV), which exhibits low differential resistance under sufficiently high applied voltage, and can therefore be useful as a shunt protection device. Another is a positive temperature coefficient polymer (PTC); this exhibits a resistance which increases rapidly in response to temperature rise, and hence can be useful for series protection against overcurrents.
A newer protection component is the transient blocking unit (TBU). The TBU is a very fast disconnection device, which can be used as a series protection device to block transient overcurrents. A TBU will typically have a much faster response time than a PTC and does not require a power source. In addition, the TBU, unlike the PTC, does not limit circuit bandwidth. TBUs are described e.g. in U.S. Pat. No. 5,742,463, in US published application US2005128669, and in published PCT applications WO2005020402, WO2004034544, WO03069753, and WO2004006408; all of these are hereby incorporated by reference.
Flexible Secondary Overcurrent Protection
The present inventions are directed towards a surge protection system which includes the ability to remotely activate surge-protecting elements, and to use a single surge-protecting element to protect equipment with different electronic thresholds.
For example, in one embodiment, a single light activated shunt protection component (e.g. a photothyristor) is placed at the connection terminal where an external line is connected to the circuit. This shunt protection component preferably is rated to crowbar at the maximum protection voltage of the circuit, even when it is not optically activated. In this embodiment, the thyristor can be adapted to be triggered into conduction through light activation. Light emitting devices can be placed at other locations within the circuit, including locations that are not always connected to the external line. In this way, many nodes of the circuit may be protected with a single protection device. If the voltage at any one of these nodes reaches its defined trip point, the LED turns on and the protection thyristor is thereby activated.
In another embodiment, the light activated shunt device is combined with a transient blocking unit (TBU) as a series device. Since the TBU is a fast switching device, the current handling requirements of the shunt device (e.g. thyristor) can be optimized, and the thyristor need only be capable of handling the peak current surge which can pass the TBU. This provides secondary advantages, including the possibility to make the device very small, sensitive, and cost effective.
Advantages of the disclosed innovations include, in various embodiments, some or all of the following:                Only one set of the relatively expensive protection components is required.        Different overvoltage levels can easily be assigned for different internal nodes of a circuit, with only a small increase in cost and complexity.        Overvoltage protection with different slew rate dependences can be used for different internal nodes of a circuit, without any increase in cost and complexity.        Overvoltage monitoring can easily be added to many internal nodes of system, with only a small increase in cost and complexity.        Line loading by the parasitic capacitance of the shunt protection device can be minimized.        Line loading by the shunt protection device is constant despite variation in the number of secondary nodes which need to be protected.        A given basic configuration of protection components can be used in a variety of system configurations.        The size of the shunt protection component can be optimized.        The whole subsystem can readily be decoupled from its inputs for internal test operations.        
Telecomm system applications, such as SLIC boards, are particularly advantageous.